Leaders have the opportunity to unleash women's full potential.

Many leaders from government and civil society will meet in London today to discuss family planning. What they discuss will have an impact on women and girls around the world.

Today more than 25,000 girls under the age of 18 will get married, and the same number will do so tomorrow, the next day, and the next. For the majority of these girls, pregnancy and childbirth will soon follow.

Complications in pregnancy and childbirth are the leading causes of death among adolescent girls aged 15-19 in low and middle-income countries, resulting in thousands of deaths every year. That so many women die as a result of complications from pregnancy or childbirth in the 21st century is a shocking indictment of the low priority given to the needs and status of women and girls in many societies.

As well, many suffer complications in childbirth. In the case of fistula, women may be rejected by their husbands and families and outcast by their communities. They may endure intense shame and physical pain.

Access to reproductive health services is a health issue, but it is also an issue of social justice and human development. It is a basic human right for women to enjoy full legal equality and equality of opportunity, and for a girl born today, in any country, to have the same life prospects as any boy.

When women’s needs for family planning and reproductive and sexual health services go unmet, their chances of finishing their education, entering and remaining in productive work, and breaking out of poverty are sharply reduced.

All too often, women and girls are discriminated against not only in access to health services, but also in education and in the labour market — with negative repercussions for not only their own freedoms, but for progress in their countries as a whole. In 2010, UNDP introduced the Gender Inequality Index as part of its Human Development Reports, to reveal differences in the distribution of human development achievements between women and men so that policymakers know to take steps to reduce disparities.

Empowering women and girls is one of the strongest tools available to accelerate development. Sexual and reproductive health and rights are an essential component of that empowerment.

When women have control over their health and sexuality, they can plan their pregnancy and childbirth, better protect themselves from HIV and other sexually transmitted infections, and fare better in their families, households, communities, and countries.

This summit, hosted by the UK government and the Bill & Melinda Gates Foundation, with UNFPA and other partners, could mark the start of a new drive to empower women and girls.

The Guttmacher Institute and UNFPA estimate that meeting the unmet need for modern family planning methods in developing countries would cost about $8.1 billion annually. With $4 billion of this total now being invested, there is a shortfall of $4.1 billion.

Leaders have the opportunity to help plug the $4.1 billion a year funding gap in order to get contraception to the hundreds of millions of women who would like to access family planning, but can’t.

Leaders can also take this opportunity to ensure that women’s and girls’ rights are enshrined in law, empowering them to decide whether, when, and how many children they have.

When women can access family planning, they have the opportunity to shape their own future, and that of their children. The benefits are felt across whole nations.

All our societies are the poorer if they fail to tap the full potential of half their population, and do not remove the obstacles to their success.

Helen Clark is the Administrator of the United Nations Development Programme and the former Prime Minister of New Zealand